Hey Rally sorry to read about your black, have your noticed any white stringy fecies this usually is common with internal parasites. Also saw the same eroding of the fins with internal parasites. This to could be why the other fish in the tank are not effected. I know alot of people have been suggesting lowering the temp, but with a w/d it seems to me the water would be oxygenated enough, plus you would see signs from the other fish if oxygen was low.
Treating internals parasites can be done with Metronidazole for flagellates and Prazipro is good for external parasites but will work for internal tapeworm.
Also high temperature 85 degrees plus. Unfortunately all my parasite dealings have been with discus, but I would imagine there is some similarities with your Black aro
Good luck with this, the link below is where I've have gotten the meds to treat my fish.
http://rockymountaindiscus.com/Food_and_medications.htm
Treating internals parasites can be done with Metronidazole for flagellates and Prazipro is good for external parasites but will work for internal tapeworm.
Also high temperature 85 degrees plus. Unfortunately all my parasite dealings have been with discus, but I would imagine there is some similarities with your Black aro
Good luck with this, the link below is where I've have gotten the meds to treat my fish.
http://rockymountaindiscus.com/Food_and_medications.htm